Optical coherence tomography (OCT) is an established in vivo optical imaging technology that provides micrometer resolution and millimeter penetration depth in human tissues. It has been validated clinically that OCT enables in vivo visualization of human eyes, coronary arteries, gastrointestinal tracts, and airway tissues at a resolution comparable to histology. OCT has been widely used clinically to diagnose a wide range of diseases in the retina and the anterior segment of the eye. Recently, intracoronary OCT technology has been used clinically to image the coronary artery disease, and endoscopic OCT technology used for detection of gastrointestinal neoplasm.
Ophthalmology is the dominant OCT application. In 2010, the estimated OCT charges in U.S.A and worldwide were $780 M and $1 B respectively, and it is estimated that the sale of ophthalmologic OCT systems was approximately $250 million. There are more than 10 suppliers of ophthalmic OCT products, where one such supplier alone delivered over 10,000 ophthalmic OCT units worth $0.7 B to the market by 2008. Over the next four years, the OCT market is expected to grow annually at approximately 60%.
Since its invention in 1991, OCT technology has evolved from a time-domain OCT (TD-OCT, first generation technology) to a spectral-domain/Fourier-domain OCT (second generation technology). An existing spectral-domain OCT (SD-OCT) setup 100 is illustrated in FIG. 1, showing a small-source SD-OCT setup with a light source (LS) 102. Based on the laser safety regulations provided in IEC (International Electrotechnical Commission) 60825, a “small source” is defined as a source with an angular subtense, α, less than, or equal to, the minimum angular subtense, αmin (generally defined to be 1.5 mrad). The angular subtense, α, may be defined as a visual angle subtended by a source at the eye of an observer or at the point of measurement. The angular subtense, α, may also be defined as the angle subtended by an apparent source (defined as the real or virtual object that forms the smallest possible retinal image) as viewed at a point in space, e.g. at the viewer's eye.
In the small-source SD-OCT setup 100, an output light 104 of the light source (LS) 102 is divided by a beam splitter 106 into a sample light radiation 108 and a reference light radiation 110. The sample light radiation 108 propagates through a sample arm (S) 120 of the setup 100 while the reference light radiation 110 propagates through a reference arm (R) 150 of the setup 100, where the sample arm (S) 120 and the reference arm (R) 150 define an interferometer.
The sample light radiation 108 is directed or guided to a tissue sample 190 under investigation through focusing optics, e.g. a collimation lens (L1) 122 and a focusing lens (L2) 124 and one or more beam scanners (SC) 126 in the sample arm (S) 120. The sample light radiation 108 is directed to a spot 192 on the tissue sample 190. A reflected and/or backscattered light radiation 109 is generated from the interaction between the sample light radiation 108 and the tissue sample 190 at all the depths the sample light radiation 108 interacts with the tissue sample 190 along an axial line (A-line) defined between the surface and the bulk of the tissue sample 190. The reflected and/or backscattered light radiation 109 is directed or guided by the same focusing optics of L1 122 and L2 124 and the beam scanner(s) (SC) 126 of the sample arm (S) 120 towards the beam splitter 106, in a direction opposite to the propagating direction of the sample light radiation 108.
The reference light radiation 110 is directed or guided to a reference mirror (RM) 160 through focusing optics, e.g. a pair of lens 152, 154, and a reflected light radiation 111 is generated, as a result of reflection from the reference mirror (RM) 160, and directed or guided towards the beam splitter 106. The back reflected light radiations 111 from the reference arm (R) 150 and the back reflected and/or back scattered light radiation 109 from the sample arm (S) 120 are recombined through the beam splitter 106 to form a spectral interference signal 112 and guided to a spectrometer 170. The spectrometer 170 includes a grating 172, a lens 174 and a detecting element (e.g. a camera) 176. The spectral interference signal 112 is recorded by the spectrometer 170 and processed by a computer (not shown).
The axial line profile (A-line) of the tissue sample 190 can be retrieved by Fourier transform of the spectral interference signal 112. A two-dimensional (2D) cross-sectional image of the tissue sample 190 can be obtained by transversely scanning the sample light radiation 108 using the beam scanner (SC) 126 while continuously acquiring axial line profiles (A-lines). A three-dimensional (3D) image can be obtained by transversely scanning the sample light radiation 108 using 2-axis (X and Y) scanners.
All existing laser scanning OCT systems use spot sources, which are small-sources. Although the existing SD-OCT technology provides a few orders of magnitude higher sensitivity than the TD-OCT technology, thereby enabling higher penetration depth and/or faster imaging speed, the ability of OCT to provide diagnostic information is still limited by its sensitivity, especially when the radiant exposure/irradiance applied to human tissues is restricted to a maximum permissible exposure (MPE) by laser safety regulations such as ANSI (American National Standards Institute) Z136 in the United States and IEC (International Electrotechnical Commission) 60825 internationally.